1. Field of Invention
The invention relates to printed circuit board surface mounted electronic chip packaging and more particularly to decreasing crosstalk or electromagnetic feedthrough in leadless ceramic chip carriers for surface acoustic wave filters.
2. Description of Background Art
Surface mount technology offers several advantages for electronic equipment manufacturers including the economic advantage of increased component packing density. As described in U.S. Pat. No. 5,369,551 to Gore et al., surface mount technology carries with it the problem of thermal mismatch between the surface mount package and the printed circuit (PC) board. In particular, the thermal mismatch problems are especially acute for relatively large leadless chip carrier in which the distance between soldering pads is also relatively large.
In addition to the thermal mismatch problems, electromagnetic feedthrough, also referred to as crosstalk, between input and output circuits create undesired effects and can be one of the most troublesome sources of interference in SAW devices. By way of example, if in a design of a SAW filter, crosstalk is not sufficiently suppressed, the desired frequency response of the filter will not be achieved. Crosstalk or electromagnetic feedthrough in a SAW filter relates to the direct coupling of an input signal from input to output interdigital transducer, in the form of electromagnetic radiation. Typically, in a SAW device, crosstalk is suppressed by providing grounding and RF shielding between the input and output transducers to reduce and preferably prevent inductive or capacitive coupling in the SAW filter.
With the typically tight arrangement of SAW device and package elements, conductors proximate to each other, yet not hard wired or intended to communicate with each other, will be coupled to a degree sufficient to disrupt the intended operation of the device. Further, in leadless chip ceramic carriers (LCCC) typical in the art, grounding between the seal ring at the upper end of a package and the customer common pads on the package bottom side does not provide a sufficient ground path to adequately suppress crosstalk and thus SAW device performance is adversely affected. In some cases, crosstalk is actually enhanced by an inadequate ground path. By way of example, in the manufacturing of well known multilayer ceramic chip carriers, ground connections are made from the seal ring to the bottom side of the soldering ground pads through only the four corners of the carrier, by going through the two ceramic top layers and connecting to plated castellations on the bottom or final layer. This series and parallel combination styled grounding path does not provide sufficient RF grounding for the SAW devices earlier described.
In spite of efforts made to date, there exists a need in the packaging art, especially for SAW device surface mount packages, to provide an improved surface mount package whereby crosstalk suppression is sufficient to obtain the desired performance from the SAW device being packaged.